The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a substrate carrier head and retaining ring of a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the surface of the substrate on which deposition occurs, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier (polishing head). The exposed surface (the lower surface as when the substrate is held in the polishing head) of the substrate is placed against a rotating polishing pad. The polishing pad may be a "standard" pad in which the polishing pad surface is a durable roughened surface, or may be a fixed abrasive pad in which abrasive particles are held in a containment media. The polishing head provides a controllable load, i.e., force, on the substrate which pushes the substrate against the polishing pad. A polishing slurry is supplied to the polishing pad. The slurry includes at least one chemically-reactive agent, and, if a standard pad is used, includes abrasive particles is supplied to the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the polishing pad.
A reoccurring problem in CMP is the so-called "edge-effect", i.e., the tendency for the edge of the substrate to be polished at a different rate than the center of the substrate. The edge effect typically results in over-polishing (the removal of too much material from the substrate) of the perimeter portion, e.g., the outermost five to ten millimeters, of the substrate. The over-polishing of the substrate perimeter reduces the overall flatness of the substrate, makes the edge of the substrate unsuitable for use in integrated circuits, and decreases the yield.
In view of the foregoing, there is a need for a chemical mechanical polishing apparatus which provides the desired surface flatness and finish while minimizing the edge effect.